Guard structures are commonly used in semiconductor devices to control the electric potential at various regions of the semiconductor chip. They are the more important the higher voltages are used on the chip. As an example of a semiconductor where relatively high voltages exist on the chip we will consider a semiconductor radiation detector.
FIG. 1 illustrates schematically a simplified, partially cut-out silicon drift detector (SDD), which is an example of a semiconductor radiation detector used to detect electromagnetic radiation, particularly X-rays. A bulk layer 101 of semiconductor material receives and absorbs the radiation, which causes free charge carriers to appear. One surface of the bulk layer 101 comprises an arrangement of concentric p-type implantation rings, of which ring 102 is shown as an example. The concentric rings are arranged to have electric potentials of gradually increasing absolute value, so that if the potential at the center of the SDD is close to zero, the outermost ring may have a potential of e.g. −150 V. The number of rings is overly simplified in FIG. 1; in a real-life detector there may be dozens of rings.
Together with a cathode layer 103 on the opposite surface of the bulk layer, the concentric rings create, within the bulk layer, an internal electric field that drives the radiation-induced electrons towards that part of the SDD where a potential energy minimum for the signal charges is located. At or close to the center, an anode is located for collecting the radiation-induced electrons. The SDD of FIG. 1 comprises an integrated field-effect transistor (FET), the electrodes of which are shown as implantations 104, 105, and 106. The innermost implantation ring, i.e. the one closest to the FET, is the anode, from which a connection 107 is made to the gate of the FET. Alternative structures are known, in which the anode is at the very center of the SDD, and an external FET is coupled to the anode for example by bonding a separate FET chip to appropriate parts of the SDD chip.
A circular SDD with the anode and FET at or close to the center of the SDD chip has the inherent disadvantage that some of the measured radiation will hit the FET, which may disturb its operation and cause radiation damage to the crystalline material from which the FET is made. In a structure like that in FIG. 1, the FET will also reserve some active surface area. As an alternative, the so-called droplet-formed detector, also known as SD3 or SDDD (Silicon Drift Detector Droplet) has been proposed. FIG. 2 illustrates schematically the surface of a droplet-formed detector, again with the number and relative size of the structural elements deliberately distorted in favor of graphical clarity. The implantation rings, the stepwise increasing potential of which create the electric field, are asymmetric so that their arched form is relatively wide on one side (on the left in FIG. 2) but narrow and pointed on the other (on the right in FIG. 2). The outermost implantation ring used for this purpose is shown as 201.
The anode region is generally shown as 202 and it may comprise conductive patches (like in FIG. 2) for bonding an external FET thereto, and/or implantations that at least partly constitute an integrated detection and amplification element such as a FET. The asymmetric form of the implantation rings brings the anode region 202 out of the active area of the detector, so it is much less exposed to radiation than in the structure of FIG. 1, and also does not cause any dead zone in detection.
According to the basic principle of drift detectors, the outermost implantation ring 201 has the highest absolute value of electric potential. In order to controllably reduce the absolute value of electric potential towards the edge 203 of the detector chip, guard rings encircle the implantation rings. FIG. 2 shows specifically an inner 204 and an outer 205 guard ring, but the number of guard rings may be anything from one to about a dozen. The guard rings may be made by implantation, and/or they may comprise trenches milled to the semiconductor material and/or conductor strips produced on its surface. The guard rings may be left floating, or a specifically selected electric potential may be coupled to each guard ring through a contact; in FIG. 2 the contacts 206 and 207 are shown as examples. The guard rings with their automatically assumed or selected potentials reduce the risk of electric breakdown and—even more importantly—help to shape the electric field inside the semiconductor material in such a way that the active volume increases.
The small graph at the top of FIG. 2 illustrates the change in electric potential along the arrow 208 that goes perpendicularly across the outermost implantation ring 201 and the two guard rings 204 and 205. At the ring proper, electric potential is constant (a large negative potential VHV for the implantation ring 201, and potentials VG1 and VG2 with stepwise decreasing absolute values for the inner and outer guard rings respectively). Between rings there is a zone of gradually changing electric potential. Here we assume that the very edge 203 of the detector chip is grounded.
There are certain drawbacks in the known semiconductor detector structures. Conductor tracks, which are not shown in e.g. FIG. 2 but which are needed to make electric contacts to the inner parts of the ring structure, must be taken across underlying zones of relatively large (by absolute value) electric potential, which may make manufacturing complicated. There are also the zones between the implantation rings, at which surface-generated currents occur. Special arrangements are needed in order to draw out those surface-generated currents so that they do not mix with the measurement of the radiation-induced signal charge.